This application claims the priority of Korean Patent Application No. 2003-77791 filed on Nov. 4, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor light emitting device using a nitride compound semiconductor, and more particularly, to a high reflectance film electrode structure that has low contact resistance and high reflectance.
2. Description of the Related Art
Generally, nitride compound semiconductors are widely used for visible light emitting devices, and are currently developed to a light emitting device for producing light in the ultra violet region since a light emitting device for producing light in the blue and green light regions has been developed. Also, the nitride compound semiconductors can be used for light sources used in light emitting devices emitting light in a blue, green and ultraviolet region, and in high density light recording devices.
As information recording density increases, a group III-V nitride semiconductor becomes more useful because the such a nitride semiconductor is capable of emitting a visible light laser and has high emission efficiency since a transition in the nitride compound semiconductor producing the light is a direct transition, and moreover, the nitride semiconductor is capable of emitting a blue light that is one of the three primary lights.
It is advantageous that a light emitting device has a low operating voltage. A method widely used for reducing an operating voltage of a light emitting device includes reducing the resistance of a material layer formed between an electrode layer and an active layer. Particularly, to reduce the operating voltage in the nitride compound semiconductor light emitting device, it is desirable to form a low ohmic resistance between a hole injecting layer and a p-type electrode since the hole injecting layer and the p-type electrode are in ohmic contact.
That is, a draw back of the ohmic contact to the p-type electrode is that there is no metal having a larger work function than the p-type GaN. Therefore, an ohmic contact to a p-type GaN is difficult. In order to manufacture a laser diode or a light emitting device having high quality using GaN, an ohmic contact having high thermal stability and low contact resistance must be formed on the p-type GaN.
A cross-sectional view of an electrode structure applied to a conventional nitride semiconductor light emitting device, disclosed in Japanese Patent Laid-Open publication 10-303504 is depicted in FIG. 1.
Referring to FIG. 1, an electrode structure 10 at an interface of a p-type GaN used for a conventional nitride semiconductor light emitting device comprises a sapphire substrate 12, a p-type GaN layer 14, a compound layer 16, and a metal layer 18 stacked sequentially.
In the conventional electrode structure 10, the compound layer 16 is formed by following process. First, the p-type GaN layer 14 is formed on the sapphire substrate 12 using a molecular beam epitaxy (MBE) method, and a surface of the p-type GaN layer 14 is cleaned using ultrasonic waves in acetone or alcohol.
Next, the wafer is heated to 200° C. to increase a bonding strength of the layers to be deposited in vacuum state, a metal layer 18 for electrode is formed using a sputtering method with argon plasma, and the metal layer is annealed for 5 minutes at a temperature of 600° C. in a nitrogen atmosphere. As a result, a compound reaction occurs at the interface between the p-type GaN layer 14 and the metal layer 18, and a compound layer 16 composed of PdGa, Pd2Ga3 or Pd3Ga2 is formed.
The conventional electrode structure described above can be a low resistance p-type electrode structure composed of an Au/Pd/Pd—Ga compound suitable for a group III nitride semiconductor device. However, when manufacturing a group III nitride semiconductor device using the p-type electrode structure, there is a drawback in that a surface of the electrode can become very rough.
Therefore, a contact resistance in the electrode surface is not uniform, increases in some regions resulting in an increase of the overall contact resistance, thereby causing an increase of the operating voltage of the device.
Also, the rough surface of the electrode can be a cause of reducing the bonding strength for stacking and assembling bonding metals in a subsequent process, thereby reducing overall yield.